False FF Readings

Fishfinders locate fish by sending out a signal that bounces off of anything that is a different density than water. So, they do not locate fish, they locate air bubbles, rocks and maybe cartilage. Salmon look like dots, bait fish look like lots of dots.
Fish that have big air sacks make arcs. I don't do much freshwater fishing but I think Bass and stripers have air sacs.

Essentially, fish are pretty much the same density as water. Also, there are places in the ocean and lakes where gas bubbles rise up that cause the fishfinder to register. It may even be that the bubbles dissipate before they make it to the surface.